One step in the fabrication of modem electronic devices such as printed wiring boards (PWBs) is the use of solder flux, normally a rosin, to insure good bonding of solder elements to copper conductors of the PWBs. After the soldering operation, residual rosin flux must be cleaned from the PWB, and for years this was done by using cleaning solvents comprising chlorinated hydrocarbons or chlorinated fluorocarbons (CFCs). The patents of Hayes et al., U.S. Pat. No. 4,740,247 and U.S. Pat. No. 4,640,719, incorporated herein by reference, point out that such solvents are harmful to the environment, and that residual rosin flux can effectively be removed by a cleaning solvent having terpene as its active ingredient, which in turn can be rinsed with water. Terpene compositions are biodegradable, non-toxic and are essentially harmless to the environment.
A system for cleaning PWBs with terpene compositions is described in the paper, "Aqueous and Semi-Aqueous Cleaning Processes," G. M Wenger et al., AT&T Technical Journal, Vol. 71, No. 2, March/April 1992, pp. 45-51, incorporated herein by reference. It is preferred that the PWBs be submerged in the terpene composition prior to rinsing, and apparatus for accomplishing this is described. PWBs that have been cleaned as described in the Wenger et al. paper have unfortunately been found to be susceptible to failure. Microscopic examinations of the component printed circuits, bonding pads and solder elements have shown that the failures are due to corrosion which, for example, may cause a solder element to break, thereby forming an open circuit. It is of course important that the causes of such corrosion be isolated and reduced or eliminated.